module Text.ParserCombinators.PolyState
( -- * A Parser datatype parameterised on arbitrary token type and state type
Parser(P) -- datatype, instance of: Functor, Monad
, runParser -- :: Parser s t a -> s -> [t] -> (Either String a, s, [t])
, failBad -- :: String -> Parser s t a
, commit -- :: Parser s t a -> Parser s t a
-- * Combinators
-- ** primitives
, next -- :: Parser s t t
, satisfy -- :: (t->Bool) -> Parser s t t
, apply -- :: Parser t (a->b) -> Parser s t a -> Parser s t b
, discard -- :: Parser s t a -> Parser s t b -> Parser s t a
-- ** error-handling
, adjustErr -- :: Parser s t a -> (String->String) -> Parser s t a
, adjustErrBad-- :: Parser s t a -> (String->String) -> Parser s t a
, indent -- :: Int -> String -> String
-- ** choices
, onFail -- :: Parser s t a -> Parser s t a -> Parser s t a
, oneOf -- :: [Parser s t a] -> Parser s t a
, oneOf' -- :: [(String, Parser s t a)] -> Parser s t a
-- ** sequences
, many -- :: Parser s t a -> Parser s t [a]
, many1 -- :: Parser s t a -> Parser s t [a]
, sepBy -- :: Parser s t a -> Parser s t sep -> Parser s t [a]
, sepBy1 -- :: Parser s t a -> Parser s t sep -> Parser s t [a]
, bracketSep -- :: Parser s t bra -> Parser s t sep -> Parser s t ket
-- -> Parser s t a -> Parser s t [a]
, bracket -- :: Parser s t bra -> Parser s t ket -> Parser s t a
-- -> Parser s t a
, manyFinally -- :: Parser s t a -> Parser s t z -> Parser s t [a]
-- ** state-handling
, stUpdate -- :: (s->s) -> Parser s t ()
, stQuery -- :: (s->a) -> Parser s t a
, stGet -- :: Parser s t s
-- ** re-parsing
, reparse -- :: [t] -> Parser s t ()
) where
-- | The @Parser@ datatype is a fairly generic parsing monad with error
-- reporting and a running state. It can be used for arbitrary token
-- types, not just String input.
newtype Parser s t a = P (s -> [t] -> (EitherE String a, s, [t]))
-- | A return type like Either, that distinguishes not only between
-- right and wrong answers, but also had gradations of wrongness.
type EitherE a b = Either (Bool,a) b
-- | Apply a parser to an initial state and input token sequence.
runParser :: Parser s t a -> s -> [t] -> (Either String a, s, [t])
runParser (P p) s =
(\ (e,s,ts)-> (case e of Left (_,m)->Left m; Right m->Right m
,s,ts))
. p s
instance Functor (Parser s t) where
fmap f (P p) = P (\s ts-> case p s ts of
(Left msg, s', ts') -> (Left msg, s', ts')
(Right x, s', ts') -> (Right (f x), s', ts'))
instance Monad (Parser s t) where
return x = P (\s ts-> (Right x, s, ts))
(P f) >>= g = P (\s ts-> case f s ts of
(Left msg, s', ts') -> (Left msg, s', ts')
(Right x, s', ts') -> let (P g') = g x
in g' s' ts')
fail msg = P (\s ts-> (Left (False,msg), s, ts))
-- | When a simple fail is not strong enough, use failBad for emphasis.
-- An emphasised (severe) error can propagate out through choice operators.
failBad :: String -> Parser s t a
failBad msg = P (\s ts-> (Left (True,msg), s, ts))
-- | Commit is a way of raising the severity of any errors found within
-- its argument. Used in the middle of a parser definition, it means that
-- any operations prior to commitment fail softly, but after commitment,
-- they fail hard.
commit :: Parser s t a -> Parser s t a
commit (P p) = P (\s ts-> case p s ts of
(Left (_,e), s', ts') -> (Left (True,e), s', ts')
right -> right )
-- Combinators
-- | One token
next :: Parser s t t
next = P (\s ts-> case ts of
[] -> (Left (False,"Ran out of input (EOF)"), s, [])
(t:ts') -> (Right t, s, ts') )
-- | One token satifying a predicate
satisfy :: (t->Bool) -> Parser s t t
satisfy p = do{ x <- next
; if p x then return x else fail "Parse.satisfy: failed"
}
infixl 3 `apply`
-- | Apply a parsed function to a parsed value
apply :: Parser s t (a->b) -> Parser s t a -> Parser s t b
pf `apply` px = do { f <- pf; x <- px; return (f x) }
infixl 3 `discard`
-- | @x `discard` y@ parses both x and y, but discards the result of y
discard :: Parser s t a -> Parser s t b -> Parser s t a
px `discard` py = do { x <- px; _ <- py; return x }
-- | @p `adjustErr` f@ applies the transformation @f@ to any error message
-- generated in @p@, having no effect if @p@ succeeds.
adjustErr :: Parser s t a -> (String->String) -> Parser s t a
(P p) `adjustErr` f =
P (\s ts-> case p s ts of
(Left (b,msg), s', ts') -> (Left (b,(f msg)), s, ts')
right -> right )
-- | @adjustErrBad@ is just like @adjustErr@ except it also raises the
-- severity of the error.
adjustErrBad :: Parser s t a -> (String->String) -> Parser s t a
-- p `adjustErrBad` f = commit (p `adjustErr` f)
(P p) `adjustErrBad` f =
P (\s ts-> case p s ts of
(Left (_,msg), s', ts') -> (Left (True,(f msg)), s, ts')
right -> right )
infixl 6 `onFail` -- not sure about precedence 6?
-- | @p `onFail` q@ means parse p unless p fails in which case parse q instead.
-- Can be chained together to give multiple attempts to parse something.
-- (Note that q could itself be a failing parser, e.g. to change the error
-- message from that defined in p to something different.)
-- However, a severe failure in p cannot be ignored.
onFail :: Parser s t a -> Parser s t a -> Parser s t a
(P p) `onFail` (P q) = P (\s ts-> case p s ts of
r@(Left (True,_), _, _) -> r
(Left _, _, _) -> q s ts
right -> right )
-- | Parse the first alternative in the list that succeeds.
oneOf :: [Parser s t a] -> Parser s t a
oneOf [] = fail ("Failed to parse any of the possible choices")
oneOf (p:ps) = p `onFail` oneOf ps
-- | Parse the first alternative that succeeds, but if none succeed,
-- report only the severe errors, and if none of those, then report
-- all the soft errors.
oneOf' :: [(String, Parser s t a)] -> Parser s t a
oneOf' = accum []
where accum errs [] =
case filter isBad errs of
[] -> fail ("failed to parse any of the possible choices:\n"
++indent 2 (concatMap showErr (reverse errs)))
[(_,(_,e))] -> failBad e
es -> failBad ("one of the following failures occurred:\n"
++indent 2 (concatMap showErr (reverse es)))
accum errs ((e,P p):ps) =
P (\u ts-> case p u ts of
(Left err,_,_) -> let (P p) = accum ((e,err):errs) ps
in p u ts
right -> right )
showErr (name,(_,err)) = name++":\n"++indent 2 err
isBad (_,(b,_)) = b
-- | Helper for formatting error messages: indents all lines by a fixed amount.
indent :: Int -> String -> String
indent n = unlines . map (replicate n ' ' ++) . lines
-- | 'many p' parses a list of elements with individual parser p.
-- Cannot fail, since an empty list is a valid return value.
many :: Parser s t a -> Parser s t [a]
many p = many1 p `onFail` return []
-- | Parse a non-empty list of items.
many1 :: Parser s t a -> Parser s t [a]
many1 p = do { x <- p `adjustErr` (("In a sequence:\n"++). indent 2)
; xs <- many p
; return (x:xs)
}
-- `adjustErr` ("When looking for a non-empty sequence:\n"++)
-- | Parse a list of items separated by discarded junk.
sepBy :: Parser s t a -> Parser s t sep -> Parser s t [a]
sepBy p sep = do sepBy1 p sep `onFail` return []
-- | Parse a non-empty list of items separated by discarded junk.
sepBy1 :: Parser s t a -> Parser s t sep -> Parser s t [a]
sepBy1 p sep = do { x <- p
; xs <- many (do {sep; p})
; return (x:xs)
}
`adjustErr` ("When looking for a non-empty sequence with separators:\n"++)
-- | Parse a list of items, discarding the start, end, and separator
-- items.
bracketSep :: Parser s t bra -> Parser s t sep -> Parser s t ket
-> Parser s t a -> Parser s t [a]
bracketSep open sep close p =
do { open; close; return [] }
`onFail`
do { open `adjustErr` ("Missing opening bracket:\n"++)
; x <- p `adjustErr` ("After first bracket in a group:\n"++)
; xs <- many (do {sep; p})
; close `adjustErrBad` ("When looking for closing bracket:\n"++)
; return (x:xs)
}
-- | Parse a bracketed item, discarding the brackets.
bracket :: Parser s t bra -> Parser s t ket -> Parser s t a -> Parser s t a
bracket open close p = do
do { open `adjustErr` ("Missing opening bracket:\n"++)
; x <- p
; close `adjustErrBad` ("Missing closing bracket:\n"++)
; return x
}
-- | 'manyFinally e t' parses a possibly-empty sequence of e's,
-- terminated by a t. Any parse failures could be due either to
-- a badly-formed terminator or a badly-formed element, so raise
-- both possible errors.
manyFinally :: Parser s t a -> Parser s t z -> Parser s t [a]
manyFinally p t =
do { xs <- many p
; oneOf' [ ("sequence terminator", do { t; return () } )
, ("item in a sequence", do { p; return () } )
]
; return xs
}
------------------------------------------------------------------------
-- State handling
-- | Update the internal state.
stUpdate :: (s->s) -> Parser s t ()
stUpdate f = P (\s ts-> (Right (), f s, ts))
-- | Query the internal state.
stQuery :: (s->a) -> Parser s t a
stQuery f = P (\s ts-> (Right (f s), s, ts))
-- | Deliver the entire internal state.
stGet :: Parser s t s
stGet = P (\s ts-> (Right s, s, ts))
------------------------------------------------------------------------
-- | Push some tokens back onto the front of the input stream and reparse.
-- This is useful e.g. for recursively expanding macros. When the
-- user-parser recognises a macro use, it can lookup the macro
-- expansion from the parse state, lex it, and then stuff the
-- lexed expansion back down into the parser.
reparse :: [t] -> Parser s t ()
reparse ts = P (\s inp-> (Right (), s, ts++inp))
------------------------------------------------------------------------